ath6kl: implement scheduled scan

ath6kl firmware supports scheduled scan functionality with the wow ssid
filter. But the firmware does not send any events after scan results
so I had to add a timer which notifies about new scan results.

Sched scan needs firmware version 3.2.0.6 or later. If firmware doesn't
support sched scan the driver will not enable the feature.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>